Embodiments of the present invention relate to a voltage-driving pixel unit, a driving method and an organic light emitting diode (OLED) display.
One way to achieve an OLED display of a large size is to form an active matrix substrate using thin film transistors. Such substrate comprises a pixel array defined by intersecting of gate lines and data lines. For each pixel of the pixel array, a switch transistor is provided; the gate line supplies a selecting signal to turn on the switch transistor; the data line supplies a voltage signal to a driving transistor in the pixel though the turned-on switch transistor; and the driving transistor drives the OLED in the pixel to emit light. Where the driving transistor is voltage-driven for a long time, stress effect may occur in the driving transistor, and, as a result, the threshold voltage of the driving transistor may drift and the current passing through the driving transistor may correspondingly vary. Since the brightness of the OLED is in proportion to the current, the above variation of the current passing through the driving transistor may result in an uncontrollable variation of the brightness of the OLED and further results in a deterioration of the display quality.
A circuit is designed to compensate the threshold voltage drift of the driving transistor, as shown in FIG. 1. FIG. 1 is a structural view showing a conventional voltage-driving pixel circuit. The voltage-driving pixel circuit in FIG. 1 comprises a switching transistor 201, a compensating transistor 202, a driving transistor 203 and a storage capacitor 204 which constitute a three-transistor-and-one-capacitor (3T1C) structure. In addition, the voltage-driving pixel circuit further comprises a signal line 260 for controlling the compensating transistor 202, a gate line 240, a data line 250, a power source Vdd 210 and a ground terminal Vss 220. The voltage-driving pixel circuit is used to drive an organic light emitting diode (OLED) 230 and the operation mechanism is described as follows. Before data is written into the pixel, the cathode voltage Vss is set to a low level, the data line 260 is set to a high level, the driving transistor 203 is turned on, and in this way, a voltage substantially equal to the threshold voltage of the driving transistor 203 is established and temporarily stored in the storage capacitor 204. During the data is written, the data line 260 is set to a low level, the data signal voltage is written (transferred) to the node A, and in this way, the voltage across the storage capacitor 204 becomes Vdata+Vth. Next, during sequence for display driving, the cathode voltage Vss of the OLED 230 is set to a low level so that the driving transistor 203 operates in the current saturation region. Because the driving transistor for the OLED 230 operates in the current saturation region, the current passing through the driving transistor is proportional to (Vgs−Vth)2, i.e., I∝(Vgs−Vth)2, where Vgs is the voltage drop between the gate electrode and the source electrode of the driving transistor and Vth is the threshold voltage of the driving transistor. In addition, when Vgs is equal to the sum of the written data signal voltage (Vdata) and the threshold voltage (Vth), I∝(Vgs−Vth)2=(Vdata+Vth−Vth)2=Vdata2, that is, the current for driving the OLED becomes independent of the threshold voltage. Thus, the drift of the threshold voltage can be compensated.
However, the above-described voltage-driving pixel circuit has the following disadvantage. During the data is written, the driving transistor 203 is in the turned-on state so that the node B is charged and reaches a high level, and thus the voltage across the storage capacitor 204 is decreased; that is, the voltage that is previously equal to the threshold voltage and stored in the storage capacitor before the data is written is decreased. Thus, the effect of compensating the drift of the driving transistor threshold voltage is degraded. Therefore, the current for driving the OLED 230 may still vary, and correspondingly, the brightness of the OLED may vary and the display quality may be deteriorated.